The Roman Army was organized into military units comprised of the Roman Legion and the Auxilia.  The Legion was Roman citizens, and was about 6,000 men strong.  The Auxilia was organized of non-citizens, and a large variety of kinds of troops.  During the reign of the Roman Empire, the number of Roman Legions ranged from a low of 25, to a high of about 35 legions.  The division of the legion was split into 10 cohorts, and a cohort was split into 6 centuries.  A century in the legion was 80 men, heavy infantrymen known as legionaries.  The cohorts would be supplemented by auxiliaries, which would provide cavalry.  Roman Legion2 (Luttwak, 14)

Off the estimate of 6,000 men total in a legion, the total manpower of legionary troops would be between 150,000 and 174,000.  The evidence for size of the auxilia in the empire as a whole is inconclusive, a valid approximation of auxilia would be a 1:1 ratio to the legionary forces.  The total number of Roman troops would therefore be about 300,000 with a possible maximum of 350,000 troops (Luttwak, 16).  Legionary troops were trained and paid; they were a professional force.  The auxilia were paid, but as non-citizens they were paid substantially less than legionaries.  And given the degree of specialization of the legionary forces and their tactical limitations, it is clear that the auxilia were not merely additive, but complimentary to the legions (Luttwak, 41).  Although the auxilia was certainly complimentary to the legion, the main advantage of the Roman army.  The disciplined and well-trained legionary soldiers had fighting ability that made the ancient world fear its name.

The basic strategy in combat was for the legion to fight in a concentrated mass.  The legion never gave enemies serious threats with missiles, they were limited because there were few pila.  Also, the range of a pilum would not normally exceed 100 feet (Luttwak, 42).  The cavalry units of legionary forces were not intended as heavy shock troops, their cavalry could only provide scouts and pickets.  A Roman legion would approach an enemy relatively slowly, enemies were not as heavily armed therefore faster.  They would advance slowly toward the enemy center and take them out by siege or assault.